1995 Yamaha 9.9 4-stroke dropping cylinder until warm

[trstanley54]

I've had this problem since I got the boat 3 years ago and have driven myself nuts looking for a fix. It starts every time as soon as I touch the key, no fast idle needed. It has never died and never failed to start. However, it will randomly stop firing on the lower cylinder until it's run for about 20 minutes, and then it runs fine. It doesn't stop firing all together, as it will surge for moments and then drop again. It runs as smooth as can be and there aren't any extra noises. So far I've cleaned the carb/jets, replaced plugs, new coil/wires/boots, new fuel tank/lines and still have the issue. Since it has this issue sporadically for only the first 20 minutes +/- it's hard to troubleshoot. I'm thinking it could be something with the fuel pump or cdi, but I still don't get why it comes out of the issue after being good and warm. I've spent hours reading forums to no avail, so I'm throwing in the towel and asking for help.

If it helps, the model is T9.9ELHT.

 

[rejesterd]

Hi.
Sounds like the enrichment system. That's what keeps the air/fuel mixture from being too lean when the engine is cold. Look for info on "electrothermal valve" and "valve heater coil". If you have the money, definitely buy the service manual from yamahapubs.com.

But yours might be too old for that, and it's probably just your choke is not right. Again, the service manual should tell you how to check it.

 

[trstanley54]

Buying the service manual is the logical and rational way to learn, but what's the fun in that? The engine schematics don't show it having the valve/coil, but it does have a starter set - is that similar?

 

[rejesterd]

Looks like you have a choke:

https://www.boats.net/catalog/yamaha/outboard-by-hp/9-9hp/t9-9elht-1995/carburetor